Use the command ipconfig /all for Windows
NT/2000/XP-based systems and ifconfig –a for UNIX/Mac
OS platforms. Test UTP for voltage levels, and the presence and
polarity of link pulses. Many new switches and some NICs have
auto-sensing ports that compensate for polarity faults on a
copper cable. Moving a station from a newer switch which
compensates for some types of cable fault to an older hub or
switch may give a mistaken belief that the older device is at
fault, when in fact the newer switch was compensating for a
pre-existing cable fault. Copper links between one hub and
another, between two switches, or between two PCs require
crossover connections. Stations are connected to Layers 1-2
devices with cables that are not crossed over, often called
straight cables. Check for crossover cables or hub and
switch ports that are configured as a crossover with a simple
button press. Many new switches, as well as all 1000BASE-T
links, are also able to compensate for crossover cables used
instead of straight cables. Some simple hubs use a single
connection internally to service a crossover port and an
adjacent normal port, only one of them may be used for RJ-45
jacks. This configuration is usually indicated beside the jacks
for the shared port. Split pair cables will either operate
poorly or not at all, depending on the Ethernet speed used, the
length of the split segment, and how far it is located from
either end. The further the split is located from a
transmitter, the less effect it will have due to signal
attenuation. If the split is short, like a patch cable, and it
is located midway between the two ends of a long run, 10BASE-T
may operate mostly error free. Even a short split cable
anywhere along a 1000BASE-T link will likely disrupt traffic,
and may even prevent the link from coming up.
Content
3.4 Identifying Physical Layer Problems
3.4.3 Cabling faults – fiber and coax All fiber
links are crossed over. The connectors are always the same on
stations and infrastructure equipment, so the TX output is
connected to the RX input through careful attention to the
cable polarity. Check fiber for swapped RX/TX connections when
polarized or small form factor multi-fiber connectors are not
used. Someone may have reconnected the cable incorrectly after
disconnecting for some reason. Figure shows the coaxial cable
and Figure shows the fiber-optic cable.If a spare fiber-optic
cable did not have dust covers on it, then it should be
considered suspect even if it tested good yesterday. Clean all
fiber-optic cables before attaching them as routine procedure.
If a fiber power test fails, inspect and clean all connections
on the link. While walking the cable run route, watch for
excessively tight bends and over tightened cable ties that
cause micro bends. Test fiber for power level and link
indicator LEDs. Coaxial cable
Coaxial cabling runs
form a bus topology. Each station NIC acts as an intermediary
connector for the run, linking devices in the LAN in one
continuous run. Thin Ethernet (10BASE2) can contain 30 hosts in
a LAN segment. Breaks in the cable can be difficult to locate
because all stations form one continuous segment, with no
micro-segmentation occurring. Problems with coaxial cable often
occur at the connectors. When the center conductor on the
coaxial cable end is not straight and of the correct length, a
good connection will not be achieved. When the center
conductor is not straight or is not the correct length, cut the
coaxial cable behind the connector end, and strip the
insulation back. Make sure that the newly exposed center
conductor is straight. Before replacing the new cable connector
end, check the general condition of the cable. Make sure that
the new cable conductor end is securely crimped to the cable.
The center connector should extend 3.2 mm (1/8 inch) beyond the
end of the connector. Check that the coaxial cable end is
securely screwed onto the F-connector at the back of the cable
access router. Hand-tighten the connector, making sure that it
is finger tight; then give it a 1/6 turn. The connection of a
host NIC to 10BASE2 cabling is usually via a BNC connector.
Some buildings use a special type of connector in place of the
more common BNC to try to prevent problems arising when users
disconnect or reconnect their equipment. Without the special
connector, a person unplugging their equipment could cause a
break in the transmission medium inhibiting communication
between other users. The end of each segment of cable must be
terminated with a 50 Ohm BNC terminator. Thick Ethernet
(10BASE5) cabling runs use thick coaxial cable to form a bus
topology. A maximum of 100 transceivers can be used for host
connection to the bus. Thick Ethernet cabling is difficult to
install because of its weight and large diameter of 0.5 inch.
It is not acceptable to bend 10BASE5 cabling, there are also
constraints on how tightly the cabling run can be curved. Thick
Ethernet was primarily used for LAN backbone connections and it
was uncommon to find it used to connect end-stations to the
LAN.
Content 3.4 Identifying Physical
Layer Problems 3.4.4 Hardware When Layer 1
or Layer 2 hardware components fail, a system will experience a
sudden loss of physical connectivity. There are various
occurrences in frames transported over shared access media that
indicate a faulty NIC or interface in Layers 1-3 equipment.
Check for link lights at both the station and the hub or switch
end. However, due to increased software control the presence of
a link light is not a guarantee that the port works. The
absence of a link light is still a fairly reliable indication
of a problem. Disconnect the problem station or the station
identified as the source of errored frames from the network.
Attach a monitoring tool in its place. Be sure to use the
original cable of the problem station at this time, not a
known-good cable. Test also from the network side back toward
the suspect station. Use a spare cable to attach the test tool
to the station and watch the power-up process. Without
familiarity with low level operating system commands it is
often necessary to cold-start the station in order to get it to
speak on the network. Once a station has completed the initial
boot-up process it may not speak on the network again without
these special commands. Observe the link process and the
protocols that the station is sending. Ensure that the station
is communicating, as many problems relate to station
configurations not recognizing or attempting to utilize the
network adapter. Look for signal strength and other physical
layer parameters. Not all network adapter faults will be
exhibited during testing. If the fault appears to be
intermittent it may be appropriate to replace the network
adapter and driver software as part of the diagnostic process.
If that solves the problem then try exchanging just the network
adapter to see if the fault was associated with the network
adapter or the driver software. If the network adapter was
inexpensive enough, it may be more expedient to simply discard
the suspect adapter if the user later reports that the problem
has been resolved by replacing it. Some portable PC network
adapter cards ship with a special power saving feature enabled.
This feature causes the NIC to listen for link pulse, but it
will not transmit anything at all (including link pulse) until
it hears a signal on the receive circuit. To conserve power the
entire transmit circuit in the NIC is shut down until the
receive circuit indicates that a transmission is warranted.
This feature has been known to cause link problems with some
other network devices. Try disabling this feature in the
software configuration of the NIC, or use a test tool to see
what signals are offered by the NIC.
Content
3.4 Identifying Physical Layer Problems
3.4.5 Collision based problems – shared media
Excessive collisions are most often caused by a problem with
the physical media, such as missing or incorrect terminators,
impedance discontinuities (bad connectors, cable stubs, crushed